The down-sizing of computers, especially of personal computers (hereinafter referred to as PCs), has been in progress for some time. For example, they have been down-sized from desk-top type to lap-top type, to note-book type and further to memo-book type. This has been accompanied by the down-sizing of magnetic disk devices used in PCs. For example, from 3.5 inch to 2.5 inch to 1.8 inch and further to 1.3 inch. This down-sizing trend is similar in optical disk devices as in magnetic disk devices. Hereinafter, magnetic disk devices, and optical disk devices, etc. will be generically referred to as disk devices.
Mechanical connecting/disconnecting technology and assembling technology of a detachable type disk device called card-type has appeared along with the down-sizing of disk devices and is disclosed in Japanese patent unexamined publication 5-181565 or 4-356785. The typical standard including electrical connecting interface was established in cooperation of Japan Electronic Industry Association (JEIDA) and US Personal Computer Memory Card International Association (PCMCIA) and is the standard specification for IC memory-card. Initially this standard was not intended for disk devices, but has been expanded to include disk devices during the course of down-sizing. This portable small disk device is connected to the memory-card slot of a PC directly, or indirectly through a connecting cable, and is sometimes operated in a partially or entirely exposed state. Therefore, it is important that radiation noise from the device should not exceed the standard, and to prevent damage or performance deterioration from electro-static discharge.
Furthermore, recently, Electro-Magnetic Compatibility (EMC), which relates to a device or system's capability to function satisfactorily without electro-magnetically affecting other surrounding devices, has more frequently been required for devices including information processing devices. For example, an electric device is allowed to be in the market in Europe from Jan. 1, 1996 only when it satisfies the EMC Standard and the CE-mark is indicated on it, according to EU EMC order/336. This kind of EMC control has been a world-wide trend and not limited only to the EU.
The Electrostatic Discharge (ESD) test is included in the above mentioned EMC control items. According to the international standard IEC 801-2(1991-04), due to the variation in devices and systems, it is difficult to evaluate and determine the influences of ESD by a general standard. Thus, test results are classified according to the following performance evaluation items in accordance with operating conditions and functional specifications of the tested devices.
1. Normal operation within the limit of specifications.
2. Temporary and self-recoverable deterioration or loss of function or operation.
3. Temporary deterioration or loss of function and operation, which is recovered by operator or system-reset.
4. Un-recoverable deterioration or loss of function by damage of a device or a software, or by loss of data.
There are three test conditions, a contact discharge test, an air discharge test (both of which are direct discharge tests), and an in-direct discharge test. Discharge voltages include 2, 4, 6, 8, and 15 kV.
The built-in type of small magnetic disk devices (Hard Disk Drives) which are generally to be assembled to be used in a PC etc., do not have operational functions and enclosing cases independently. Since it is difficult to evaluate the above mentioned three items for such devices, it has been authorized by the international authorization organization TUV that, after direct discharge to the handling portions and in-direct discharge to neighborhood portions of an in-active small magnetic disk device, the device is installed in a PC and operated and the above mentioned item 4 is tested. The test results are used as a part of judgment whether the device conforms to the EMC control.
However, a PC in which a small magnetic disk device is installed has to be tested with the above mentioned Electrostatic Discharge (ESD) test for items 1, 2, 3 in an active state. Further there is an operating environment that the portable small disk device itself has to be operated in an exposed condition partly or entirely. Therefore, it is necessary for the small magnetic disk device itself to obtain higher durability to Electrostatic Discharge in an active state.
Prior art countermeasures for electrostatic discharge related to portable data recording devices are disclosed in patent unexamined publications JP 60-83287 and JP 7-58501. In these prior art, technologies to obtain higher durability to electrostatic discharge in in-active state are stated, but not in active state.
In the case of active state, technologies to obtain higher durability to electrostatic discharge, are disclosed in patent unexamined publications JP 61-175991 and JP 1-299091, but are not especially effective for disk devices.
Moreover, apart from the field of portable data recording devices, in the field of data transmission, it is known to obtain higher durability to electrostatic discharge by separating an electric circuit using optical coupling means. An example of integrated circuits for separating electric circuit using optical coupling, is MAX1490A made by Maxim Integrated Products, Inc. This IC contains a bi-directional data transmitter with optical coupling and a power transmitter including a transformer and a DC to DC converter and a separation of 500 Vrms is achieved.
Here, for an example, according to IEC801-2, durability to electrostatic discharge required for small magnetic disk devices in active state is 4 kv in contact discharge test and 8 kV in in-air discharge test. Thus, a conventional magnetic disk device shown in FIG. 3 is examined. The conventional magnetic disk device includes mainly a head disk assembly (HDA) 202 in disk drive enclosure 201 and a printed circuit board (PCB) 208 having mainly electric circuits. The HDA 202 includes magnetic disks 203, magnetic heads 204, a spindle motor 205, read/write pre-amplifier 207 and a head actuator (include voice-coil motor (VCM) 206) as a head positioning mechanism.
The PCB 208 is an electric circuit that is provided with a connector 211 through which data and signals are transmitted and received to and from the host unit (not shown) and through which power is supplied. The PCB 208 controls the HDA 202 by transmitting and receiving said data and signals and supplying the power through said connector and processes data and signals between said host unit and the HDA 202. The PCB 208 includes in addition to the connector 211 for connecting with the host unit (not shown), ferrite beads 220, 223, an interface drive controller 212, a read/write channel 215, a micro-processor 216, and a spindle motor-VCM drive circuit 219. Here, 225 is the ground of the host unit, 233 is an interface line to the host unit, 229 is a DC 5V power supply line of the host unit, 226 is a PCB ground, 227 is a DC 5V power supply line for HDA 202 passed through ferrite beads 220, 228 is a HDA ground, 232 is a DC 5V power line for PCB 108 passed through ferrite beads 223, and 238 is bus line.
As shown in FIG. 3, in conventional ordinary magnetic disk device, digital circuits including interface line 233 to the host unit, system control circuit with microprocessor 216, and digital positioning servo control circuit, are mixed with analog circuits including data read circuit from disk 203 and rotating speed detecting circuit for spindle motor 205.
The digital circuit has higher anti-noise durability than an analog circuit, since the digital circuit utilizes a binary saturated level signal with relatively large amplitude. An input signal to the digital circuit is discriminated as high or low by comparing it with a threshold level. Error timing in the digital circuit is limited to the rise/fall timing of a clock pulse.
In contrast to this, the analog circuit, especially a data-read circuit has very low anti-noise durability. For example, a very small analog signal of about 0.5 mvpp amplitude is reproduced from the head 204 in this data-read circuit, and the small signal is amplified to about 100 mvpp at a pre-amplifier 207 and is finally amplified to 500 mvpp to 1 Vpp with a total gain 60 dB. Then the amplified signal is transformed into a pulse and digitized. Therefore, there is a problem that, when a comparatively small noise arises on the signal line, the ground 225, 226, 228, or power line 227, 229 by inductive, capacitive, or conductive coupling with electro-static discharge (ESD) during a data read operation, such noise is added to reproduced signal from the head 204 and becomes large due to amplification. This in turn can cause data error, hang-up of device operation, and sometimes circuit breakdown.
Since, it is similar in optical disk device as in magnetic disk device that very small analog signal of less than a few mV amplitude is reproduced from transducer, there is a problem also in optical disk device that electro-static discharge(ESD) during a data read operation sometimes causes data error, hang-up of device operation, or circuit breakdown.